Herpesviruses are associated with a variety of disease in the human host, ranging from asymptomatic infections to life threatening disease, including encephalitis and cancer. In addition to playing a direct role in cancer genesis, herpesviruses pose an increased risk to cancer patients who may develop debilitating and life- threatening herpesvirus infections. Genital herpes simplex virus (HSV) lesions, even if clinically inapparent, can facilitate the transfer of HIV-infected cells or free virus from one individual to another, and HIV infection and AIDS increase the risks of developing certain kinds of cancer. HSV-1 serves as a prototypic herpesvirus family member and provides an ideal model to understand the requirements for virus infection since it replicates well in cell culture and many reagents have been developed for its study. Herpesvirus entry into cells requires fusion of the outer envelope of the virus with a cell membrane. Spread of infection can occur via infectious virus or virus-induced cell fusion. The ultimate objective of studies described here is to understand how herpesviruses, specifically HSV-1, induce membrane fusion, the process required for viral entry and cell fusion. Requirements for HSV-induced membrane fusion include the coordinated activities of four HSV glycoproteins (gD, the hererodimeric gH/gL complex, and gB) and specific cellular receptors. Our central hypothesis is that gD, present in the viral envelope or on an infected cell surface, binds to a cellular receptor, such as nectin-1, on an uninfected cell and activates gH/gL and gB via protein-protein interactions resulting in fusion mediated by gB. In three aims, we plan to define the sequence requirements and domains contained on gD, gH/gL and gB that are required to mediate membrane fusion and to begin studies to solve the prefusion conformation of gB. The proposed studies will result in a better understanding of the basic mechanisms that are central to herpesvirus infection of cells and virus spread from cell-to-cell. The results of these studies will lead to an understanding of the mechanism of herpesvirus-induced membrane fusion and to new approaches for the prevention and treatment of herpesvirus infections.